The ABINIT Users Mailing List ( CLOSED )

[delaire@caltech.edu]

Dear Abinit users,

I have been computing phonons in bcc vanadium, but I am running into some 
difficulties and I could use some advice. I am using Abinit (sequential) 
4.6.5.

First, I computed phonon dispersions for Nb using the input files in 
/Test_phys/t11.in and with the FHI GGA-PBE pseudopotential, and these came 
out in good agreement with the literature. 
I then adapted these input files for the case of bcc Vanadium. I am also 
using the FHI GGA-PBE in this case. I performed a volume optimization and I 
am using the optimized cell parameter (5.8074 Bohr) in the phonon 
calculation. Here are the issues I am running into:

1) when using ecut=17Ha for V (same value used for Nb) and all other 
parameters identical as for Nb (same qpt list, same kpt grid, same 
tsmear-0.015Ha), I get an error (something like) "SCF convergence error, 
failed to converge after 13 restarts..." during one of the response function 
calculations.

2) I tried to change the iscf parameter from its default value to iscf=3 
(that was the setting to compute fcc Al phonons in workshop tutorial), but 
this makes the SCF cycle diverge (goes through all nsteps, but delta_E is 
something like 10^65...!)

3) when increasing the value of ecut from 17Ha to 26Ha (which seems more 
appropriate for V based on GS energy convergence study), with the default 
iscf=5 (and all other parameters identical to t11.in), the response-function 
calculations converge ok, if I raise nstep to 50. But the phonon dispersions 
display some anomalous behavior with transverse acoustic branches showing 
begative slopes at Gamma, leading to negative frequencies from Gamma to H and 
Gamma to N.

4) I have tried to increase the kpt mesh from 8x8x8 to 16x16x16 and 
(independently) to decrease the value of tsmear from 15mHa to 5mHa, but both 
lead to a failure of the convergence (using default iscf=5) as in 1)

5) when going to iscf=3 in case 4), the SCF blows up as in 2)...

Here are my questions:
-has anyone managed to calculate good phonon dispersions/DOS for bcc vanadium?
-what parameters are likely to improve on the negative slopes/frequencies I 
am getting (case 3 above)?

Sorry for posting such a long email, and thanks in advance for any help.
Regards,
Olivier Delaire.